With a diameter that measures 43.4 meters, the dome of the Roman Pantheon ranks as the world's largest dome made of unreinforced solid concrete. Open to the weather, it allows rain to enter and fall to the floor, where it is carried away through drains. Though the opening looks small, it actually has a diameter of 8.2 m allowing it to light the building just as the sun lights the earth. The rain also keeps the building cool during the hot summer months.